1. Technical Field
The present disclosure relates to OFDMA systems and, more specifically, to a hierarchical frame structure for OFDMA systems with relay.
2. Discussion of the Related Art
Orthogonal Frequency Division Multiple Access (OFDMA) is a communication system where multiple users may communicate with a network base station by each communicating over distinct frequency ranges. The structure in which the various users communicate is dictated by a frame that is defined at the base station. FIG. 1 is a diagram showing an exemplary OFDMA frame.
The exemplary OFDMA frame 10 of FIG. 1 is a rectangular frame including a downlink (DL) subframe 11 and an uplink (UL) subframe 12 separated by a switching gap time TTG 13. The frame 10 is divided into a series of symbols, here, numbered k through K+30. Each symbol represents a unit of time. The frame includes a preamble 14 at the first unit of time, k. The preamble spans all available frequencies. The preamble is followed in time by a downlink map (DL-MAP) 15 and a frame check header (FCH) 16 for communicating how the frame is organized. A series of DL bursts 17-22 are included. Each DL burst includes a unit of communication data. Each DL burst is unique in time and frequency so that no two bursts are communicated at both the same time and same frequency. Moreover, each DL burst is communicated in one hop, for example, from a base station (BS) directly to a mobile station (MS) in the downlink (DL) and a mobile station (MS) to a base station (BS) in the uplink (UL).
The UL subframe 12 includes a ranging subchannel 23 occupying the highest frequency subchannels. The UL subframe also includes a series of UL bursts 24-28. As is the case with the DL bursts, each UL burst is communicated in one hop.
Accordingly, FIG. 1 illustrates an example OFDMA frame that may be used, for example, as a typical 802.16e frame structure, although such a frame may be used more generally for any OFDMA system and the number and relative location of the various DL and UL bursts may be changed.
A single hop is defined as a communication that is sent from a source and received directly at a destination without having to be communicated though an intermediary. A relay station (RS) is an example of a common intermediary, and communications that are sent through a single RS are considered to be communicated in two hops. Relay stations may be used to enhance the functioning range and/or signal strength of the base station by allowing for communication through one or more relay stations. FIG. 2 is a diagram illustrating a base station (BS), a first and second relay station (RS1 and RS2), and three mobile stations (MS1, MS2, and MS3). As illustrated, such an arrangement includes many possible links as the mobile stations (MS) may communicate either directly with the base station (BS) or through one or more relay stations (RS).
Conventional OFDMA systems may support multiple hops, for example, up to two hops. Where multiple hops are supported, the frame structure may divide available resources into four distinct zones that can be arranged in any order. A first-hop downlink zone may include downlink communication originating from the base station such as base station to mobile station direct communication and base station to relay station communication. A second-hop downlink zone may include downlink communication from the relay station to the mobile station. A first-hop uplink zone may include uplink communication originating from the mobile station such as the mobile station to base station direct communication and mobile station to the relay station. A second-hop uplink zone may include uplink communication from the relay station to the base station.
However, as mobile communications networks endeavor to provide more data throughput with greater reliability in a communications system including multiple relay stations, conventional approaches to OFDMA framing may fail to deliver optimum performance.